Our knowledge of characteristics of nerve and muscle in blood vessels has been derived primarily from the large conduit or capacitance vessels and, to a lesser extent, from the microcirculation. Significant disparity in these characteristics has been demononstrated among blood vessels of different regions. This research focusses on small arteries and veins, with an objective to fill the informational gap between the large and microciruclatory vessels in particular reference to the local neuroeffector mechanisms, and to delineate some special features of the pulmonary and mesenteric vessels. The features include the deficient responsiveness of smaller pulmonary arteries, the metabolic role of vascular endothelium, and the dilator innervation in the mesenteric vessels. Short arterial and venous segments as small as 0.1 mm in outer diameter will be excised from the rabbit, rat, guinea-pig, cat and dog. Newly developed sensitive pharmacological and biochemical techniques will be applied for determining the vasoconstrictor-dilator responses, transmitter uptake and release, and other related parameters. This research is a step towards our long term goal of relating the characteristics of neuroeffector component mechanisms to the functional variation among different circulatory beds and vascular segments.